Heretofore, a single channel medium PRF pulse doppler radar receiver was proposed that effectively prevented sidelobe return signals from being displayed as "ghost" or "false" targets. In such system, the received signal data is preliminarily processed by a filter bank and a CFAR threshold circuit to remove main-beam clutter and area sidelobe clutter returns. The preliminarily processed data is then temporarily stored as two parallel correlations are performed on a range cell by range cell basis to identify range cells which contain discrete sidelobe clutter return signals. One of the parallel correlations is performed on the data after it is passed through a sensitivity time control threshold circuit, and the other parallel correlation is performed on the raw data. The outputs of the parallel correlators are then compared. The identified range cell correlations in the raw data correlator are compared with corresponding range cells of the sensitivity time control correlations to identify range cell numbers which contain discrete sidelobe clutter return signals. Identified range cell numbers are then blanked from the raw data before it is correlated for a third time. The output of the third correlator identifies the true target return signals and the corresponding unambiguous range thereof.
Although such a system, which is described in detail in U.S. Pat. No. 4,095,222 entitled "Post-Detection STC In A Medium PRF Pulse Doppler Radar" issued June 13, 1978 operates effectively and satisfactorily, certain ground targets that are moving at high speed may represent a predominant source of false alarms, particularly where there are a relatively large number of such ground targets compared to airborne targets of interest.
One possible approach to reducing detection of ground moving targets is to widen the main-beam clutter notch by blanking additional fast fourier transform (FFT) filters; or in other words, by rejecting them on a doppler basis. Although such an approach would be adequate to reject high speed ground moving targets in high PRF radars, it has a substantial impact on the radar detection performance against higher speed desirable targets because doppler is ambiguous in a medium PRF mode. While multiple PRF's can reduce the blind speeds, they cannot be completely avoided when using wide main-beam clutter notches. In other words, in a medium PRF radar, if the notch is widened enough to reject all ground moving targets such as up to 80 knots, for example, blind speeds may occur at high velocities due to PRF ambiguities. For example, the wider the main-beam clutter filter notch, the fewer the number of PRF's out of a total of eight, for example, will a given target be visible. With a wider notch there will be some velocities where fewer than the three PRF's, for example which are required for a detection, are visible.
Referring to FIG. 1A, the continuous curve referred to as x represents the number of visible PRF's for an 80 knot doppler notch filter, from less than five through forty kilohertz doppler frequency. Assuming that a minimum of three PRF's is required to effectively correlate a target traveling at 80 knots or above, blind speeds will occur at 12.3 khz, 13.5 khz and 29 khz. In contrast, FIG. 1B illustrates visible PRF's for a doppler notch filter that rejects ground moving targets of approximately 55 knots or less. It is readily seen from curve Z that the minimum of three PRF's, for example, of all of the PRF's are visible for such a 55 knot doppler notch filter. Thus, all low speed targets are rejected while no high speed targets are rejected, due to blind speeds.
Therefore, it is desirable to provide a method and system that substantially reduces the high speed ground moving target detection problems while at the same time improving the high velocity visibility without blind speeds.
Now in order to detect all targets exceeding 80 knots, for example, as encompassed by this invention, there must be at least one PRF with an ambiguous velocity outside the 55-80 knot region. Referring to FIG. 1C and comparing with FIG. 1B, it is seen that curve y amplitude is less than curve Z at greater than 80 knots, thus satisfying the criteria.